Proofreading involves carefully checking a document for errors in spelling, grammar, punctuation and consistency to ensure the intended message is clear and the writing makes a good impression. It is important for both authors and businesses to proofread in order to avoid mistakes that could damage credibility or have legal consequences. The lesson teaches common proofreading techniques like preparing a distraction-free workspace, reading thoroughly for understanding and flow, and using standard proofreading marks when marking up documents for corrections.